1. Field of the Invention
The present invention relates to a ferrite material having high impedance in a high-frequency band, and a method for producing the same.
2. Description of Related Art
Various types of noise filters for inhibiting transmission of noise of electronic appliances have been proposed and employed, and as filters for inhibiting transmission of signal components in a high-frequency band, for example, noise filters obtained by winding coils on ferrite cores have been known.
For these noise filters, magnetic impedance of ferrite materials has be utilized, and along with the miniaturization of various types of electronic appliances in recent years, ferrite materials having high impedance particularly in a high-frequency band have been desired.
Conventionally, Mn—Zn-based or Ni—Zn-based ferrite materials have been employed. Since Mn—Zn-based ferrite materials have high magnetic permeability, the materials have high impedance in kHz band, however the specific resistance thereof is as low as 1 Ωm and therefore, loss due to eddy current in the ferrites is increased in MHz band. Accordingly, the impedance is lowered at frequency of 10 MHz or more and the materials are difficult to be used for removing noise in a high-frequency band of 30 MHz or more. Furthermore, since the specific resistance is low, it is also difficult to directly wind a coil on a core and production steps such as a step of inserting an insulating material between the core and the coil are increased and the miniaturization is thus difficult.
On the other hand, since Ni—Zn-based ferrites have resistivity as high as 106 Ωm or more, the effect of eddy current loss is small, and high impedance can be obtained at a frequency of 10 MHz or more as compared with the Mn—Zn-based ferrites. Therefore, the Ni—Zn-based ferrites are mainly used for removing noise in a high-frequency band of 30 MHz or more. Further, since the Ni—Zn-based ferrites have high specific resistance, the ferrites have advantages that direct winding of a coil on a core is possible and the miniaturization is also possible.
However, since the Ni—Zn-based ferrites have high magnetostriction and show large change of magnetic characteristics by external stress, if a coiling is firmed on a ferrite core and thereafter molding with a resin is carried out in order to improve the reliability, there is a problem of changing magnetic permeability by the stress. Further, since costly Ni is contained as a main component, there is another problem that production cost is high.
As a material containing no costly Ni and having small magnetostriction, Li-based ferrite materials have been known. For example, Japanese Patent Application Laid-Open No. 2004-153197 has proposed a Li—Cu—Zn ferrite having a material composition defined as Li0.5xZnzCuaFe2+0.5x-yMnyO4 (0.10<x<1.00; 0.13<y<0.80; 0<z<0.90; 0.02<a<0.40; and 0.5x+z+a≠1), so that low temperature sintering is made possible and improved specific resistance can be thus achieved.